The online DDL feature provides support for in-place table
alterations and concurrent DML. Benefits of this feature include:

Improved responsiveness and availability in busy production
environments, where making a table unavailable for minutes or
hours is not practical.

The ability to adjust the balance between performance and
concurrency during DDL operations using the
LOCK clause. See
The LOCK clause.

Less disk space usage and I/O overhead than the table-copy
method.

Typically, you do not need to do anything special to enable online
DDL. By default, MySQL performs the operation in place, as
permitted, with as little locking as possible.

You can control aspects of a DDL operation using the
ALGORITHM and LOCK clauses of
the ALTER TABLE statement. These
clauses are placed at the end of the statement, separated from the
table and column specifications by commas. For example:

The LOCK clause is useful for fine-tuning the
degree of concurrent access to the table. The
ALGORITHM clause is primarily intended for
performance comparisons and as a fallback to the older table-copying
behavior in case you encounter any issues. For example:

To avoid accidentally making the table unavailable for reads,
writes, or both, specify a clause on the
ALTER TABLE statement such as
LOCK=NONE (permit reads and writes) or
LOCK=SHARED (permit reads). The operation
halts immediately if the requested level of concurrency is not
available.

To compare performance between algorithms, run a statement with
ALGORITHM=INPLACE and
ALGORITHM=COPY. Alternatively, run a
statement with the
old_alter_table configuration
option disabled and enabled.

To avoid tying up the server with an ALTER
TABLE operation that copies the table, include
ALGORITHM=INPLACE. The statement halts
immediately if it cannot use the in-place mechanism.